There are many and varied applications of silicon (Si), each application with its own particular specifications.
Most of the world production of metallurgical grade silicon goes to the steel and automotive industries, where it is used as a crucial alloy component. Metallurgical grade silicon is a silicon of low purity. Typically, metallurgical grade silicon that is about 98% pure silicon is produced via the reaction between carbon (coal, charcoal, pet coke) and silica (SiO2) at a temperature around 1700° C. in a process known as carbothermal reduction.
A small portion of the metallurgical grade Si is diverted to the semiconductor industry for use in the production of Si wafers, etc. However, the semiconductor industry requires silicon of ultra-high purity, e.g. electronic grade silicon (EG-Si) having approximately a 99.9999999% purity (9N). Metallurgical grade silicon must be purified to produce this electronic grade. However, the purification process is elaborate resulting in the higher cost of electronic grade silicon.
The photovoltaic (PV) industry requires silicon of a relatively high degree of purity for the production of photovoltaic cells, i.e. solar cells. The purity requirements of silicon for best performance in solar cell applications are:                boron (B)<3 ppmw,        phosphorus (P)<10 ppmw,        total metallic impurities<300 ppmw and preferably <150 ppmw.        
Although the degree of silicon purity required by the photovoltaic industry is less than that of the semiconductor industry, an intermediate grade of silicon, i.e. solar grade (SoG-Si) silicon, with the necessary low boron and low phosphorus content is not readily commercially available. One current alternative is to use expensive ultra-high purity electronic grade silicon; this yields solar cells with efficiencies close to the theoretical limit but at a prohibitive price. Another alternative is to use less expensive “scrap” or off-specification supply of electronic grade silicon from the semiconductor industry. However, improvements in silicon chip productivity have resulted in a decrease in the “scrap” supply of electronic grade silicon available to the PV industry. Moreover, parallel growth of the semiconductor and photovoltaic industries has also contributed to the general short supply of electronic grade silicon.
Several methods of purifying low-grade silicon, i.e. raw silicon or metallurgical grade silicon, are known in the art.
In US 2005/0074388 it is mentioned that:
“For electronic and photovoltaic applications that require high degrees of purity, the method of producing finished products such as photoelectric cells or solar panels comprises a step to produce pure silicon from a basic material that is a silicon essentially with a low content of boron and phosphorus.
For a long time, declassified products derived from the production of electronic silicon have formed the main source of photovoltaic quality silicon, but this source is insufficient to supply the increasing market demand so that other silicon sources are necessary such as metallurgical silicon produced by carbothermal reduction of silica in a submerged electric arc furnace, for which the quality may be improved using various secondary metallurgy refining treatments, for example, refining with chlorine described in patent EP 0.720.967 (Pechiney Electrometallurgie). Thus, a silicon is produced satisfying specifications for example such as the following (% by weight):
Iron<0.30%;
Calcium<0.10%;
Aluminium<0.30%;
Boron 20 to 50 ppm;
Phosphorus 20 to 100 ppm.
The phosphorus content is very dependent on the reduction agents used. With charcoal, it is easy to obtain silicon with a phosphorus content of about 50 ppm; this type of silicon is used particularly for making silicones. With fossil reduction agents, a silicon with a phosphorus content of less than 25 ppm can be produced, for which the main application is manufacturing of aluminium-silicon alloys. However, the purity level of these two grades is still very different from the purity level required for electronic and photovoltaic applications.
Segregated solidification has been known for a long time, and can lower the contents of many impurity elements in silicon. However, this technique is not efficient in achieving the required boron and phosphorus purity levels, starting from the two grades mentioned above.
Thus, under the pressure of the increasing market demand, a large research effort was undertaken to make a silicon with a low boron and phosphorus content starting from metallurgical silicon, particularly using purification of silicon molten under plasma. These plasma treatment techniques were designed on a laboratory scale and it is difficult to transpose them to industrial scale as a result of technical difficulties encountered in making larger tools.”
Also known in the art is U.S. Pat. No. 4,094,731, which discloses a process for producing crystalline silicon having an iron concentration less than about one-twentieth of the iron concentration of the mother liquor. Iron-contaminated silicon is introduced into a mould and the mould walls are maintained at a temperature sufficient to cause silicon crystalline growth. The mother liquor is agitated with a carbon rod stirrer or by bubbling a gas to wash the exposed surfaces of the growing silicon crystals and to prevent the freezing of the top surface of the mother liquor. A hollow crystalline silicon ingot is formed and both the inner zone centrally of the crystalline ingot and the outer zone adjacent to the mould wall are removed leaving an inner zone having an iron concentration less than one-twentieth of the iron concentration of the original mother liquor. However, neither boron nor phosphorous is removed by this process.
Over the past years a lot of effort has been put into the development of a process for making solar grade silicon at a competitive cost and on a large-scale. However, there is still a need for a process for the production of solar grade silicon on a large-scale basis.